1. Field of the Invention
This invention relates to a process for producing a modified polypropylene having a high melt tension.
More particularly, it relates to a process for producing a modified polypropylene having a high melt tension, a superior stiffness and moldability and yet, capable of being used for recycle by remelting after used as a molded product.
2. Description of the Related Art
Crystalline polypropylene is superior in the mechanical properties, resistance to chemicals, etc. and very useful in the aspect of the balance between these properties and the economy; hence it has been broadly used in various molding fields. However, since it has a low melt tension and a low crystalline temperature, it has been inferior in the moldability such as hollow molding, foam molding, extrusion molding, etc.
As a process for raising the melt tension and the crystallization temperature of crystalline polypropylene, there has been disclosed a process of mixing an organic peroxide and a crosslinking auxiliary with crystalline polypropylene, followed by melt-kneading the resulting mixture by means of an extruder (Japanese patent application laid-open Nos. Sho 59-93711 and Sho 61-152754). However, the improvement of the melt tension has been insufficient.
Further, there has so far been known a process of adding a crosslinking auxiliary to polypropylene, followed by irradiating ionizing radiations to obtain a crosslinked polypropylene (Japanese patent publication No. Sho 35-13138 and Japanese patent application laid-open Nos. Sho 49-110736 and Sho 54-25972). Since the object of these techniques has been directed to formation of a tight, three-dimensional bond to thereby improve the heat resistance, a large quantity of gels were formed; hence molding processing after the crosslinking was impossible.
On the other hand, Japanese patent application Laid-Open No. Sho 62-121704 discloses a process of irradiating electron rays onto a semi-crystalline polypropylene in the absence of oxygen to obtain a polypropylene having a free end, long chain branch and containing no gel, but irradiation of a large quantity of electron rays has been required.
Problem to be Solved by the Invention
As described above, polypropylenes obtained according to known processes have raised problems that improvement in the melt adhesion thereof has been insufficient, and besides, due to gels contained therein, it has been impossible for molded products therefrom to remelt them after used and reuse them for recycle.
In order to solve the problems raised in the prior inventions and invent a polypropylene suitable to moldings such as blow molding, foam molding, extrusion molding, etc. and further a process for producing the same, the present inventors have made extensive researches. As a result, the present inventors have found a modified polypropylene having a specified structure and properties according to a process of irradiating a low dosage of ionizing radiations upon a mixture of a polypropylene with a specified quantity of a cross-linking auxiliary, and when the modified polypropylene is used for molded products, the problem of known inventions can be solved.
As apparent from the above description, the object of the present invention is to provide a process for producing a modified polypropylene having a high melt tension and a superior stiffness and moldability, and yet after used as molded products, being capable of remelting and using the molded products for recycle; and also to provide a molded product obtained by using the modified polypropylene obtained according to the above process.